Most conventional wastewater treatment plants use an activated sludge process to remove contaminants from the wastewater. Many different wastewater treatment schemes employing the activated sludge processes have been developed over the years with each of the different systems demonstrating different operating characteristics and different benefits. FIG. 1 is a table that depicts the operating characteristics of most types of wastewater treatment arrangements employing activated sludge process. Each of these wastewater treatment systems are summarized in the paragraphs that follow.
The activated sludge process is a biological wastewater treatment method in which carbonaceous organic matter of wastewater is consumed by microorganisms for life-sustaining processes (e.g. growth, reproduction, digestion, movement, etc.). The activated sludge process preferably occurs in an aerobic environment where oxygen is consumed during the utilization and degradation of organic materials and by-products of carbon dioxide and water are formed.
An activated sludge process is characterized by the suspension of micro-organisms in the wastewater, a mixture referred to as the mixed liquor. Activated sludge processes are designed based on the suspended solids within the mixed liquor (MLSS) and the organic loading of the wastewater, as represented by the biochemical oxygen demand (BOD) and/or chemical oxygen demand (COD). The MLSS represents the quantity of microorganisms involved in the treatment of the organic materials in the aeration basin, while the organic loading typically determines the requirements for the design of the aeration system.
The activated sludge process is preferably used as part of an overall wastewater treatment process or system, which includes a primary treatment of the wastewater for the removal of particulate solids prior to the use of activated sludge as a further treatment process to remove suspended and dissolved organic solids. A primary clarifier is typically used for such bulk separation
After primary treatment of the influent to separate and remove particulate solids, the influent is then sent to an aeration basin or tank where the activated sludge process is initiated, as described in more detail herein. The solution or mixed liquor exiting the aeration basin is received by a secondary clarifier or settling tank, where flocs of microorganisms with their adsorbed organic materials settle out. This separation of sludge produces clear water or effluent which is often further treated to remove harmful bacteria and subsequently discharged. To control the biological process, a portion of the sludge that settle to the bottom of the secondary clarifier are returned to the activated sludge basin and a portion is disposed as part of a waste activated sludge stream for further treatment, for example to a aerobic digester or other solids handling process (thickening, dewatering, etc.).
In a conventional activated sludge process the primary effluent and acclimated micro-organisms (activated sludge or biomass) are aerated in a basin or tank. After a sufficient aeration period, the flocculent activated sludge solids are separated from the wastewater in a secondary clarifier. The clarified wastewater flows forward for further treatment or discharge. A portion of the clarifier underflow sludge is returned to the aeration basin for mixing with the primary-treated influent to the basin and the remaining sludge is wasted to the sludge handling portion of the treatment plant. Typically, the conventional activated sludge system operates at MLSS levels of 1500 to 3000 mg/L with a dissolved oxygen level at about 1 to 3 mg/L, a solids retention time of only 5 to 15 days, a hydraulic retention time of about 4 to 8 hours, a Food to Microorganism ratio of about 0.2 to 0.4, a Volumetric Loading of about 20 to 40 pounds BOD per day per 1000 cubic feet and a recycle ratio of between about 0.25 to 0.75
In the completely-mixed activated sludge process, influent wastewater and the recycled sludge are introduced uniformly through the aeration tank. This allows for uniform oxygen demand throughout the aeration tank and adds operational stability when treating shock loads. Aeration time ranges between 3 and 6 hours. Recirculation ratios in a completely-mixed system will range from 50 to 150 percent. The complete-mix activated sludge system operates at slightly higher MLSS levels than conventional systems, typically at 2500 to 4000 mg/L with similar dissolved oxygen level at about 1 to 3 mg/L and solids retention time of 5 to 15 days. Because of the enhanced treatment capabilities of the complete mix activated sludge process, it requires an aeration time or a Hydraulic Retention Time of only 3 to 5 hours. The typical Food to Microorganism ratio is about 0.2 to 0.6, a Volumetric Loading of about 50 to 120 BOD per day per 1000 cubic feet and a Recycle Ratio of between about 0.25 to 1.0
Extended aeration activated sludge plants are designed to provide a 24-hour aeration period for low organic loadings of less than 25 pounds biochemical oxygen demand per 1,000 cubic feet of aeration tank volume. This approach requires the use of very large basins which increases the aeration time and associated aeration power costs. Typically, the extended aeration activated sludge process operates at MLSS levels of 3000 to 6000 mg/L with a low dissolved oxygen level at about 1 to 3 mg/L, a high solids retention time of 20 to 30 days. The extended aeration process is also characterized by a very long hydraulic retention time of about 18 to 36 hours, a low Food to Microorganism ratio of about 0.05 to 0.15, a low Volumetric Loading of about 10 to 25 pounds of BOD per day per 1000 cubic feet and a Recycle Ratio of between about 0.5 to 1.50.
The closed-loop reactor, also known as an oxidation ditch, is a form of the extended aeration process. In an oxidation ditch process, the wastewater is propelled around a large area, oval racetrack-configured basin by mechanical aerator/mixing devices located at one or more points along the basin. These devices typically are either brush aerators, surface aerators or jet aerators. The velocity in the basin is designed to be between 0.8 and 1.2 feet per second. Much like the extended aeration process, the oxidation ditch process typically operates at MLSS levels of 3000 to 6000 mg/L with a low dissolved oxygen level at about 1 to 3 mg/L, a high solids retention time of between 10 and 30 days. The oxidation ditch process is also characterized by a very long hydraulic retention time of about 8 to 36 hours, a Food to Microorganism ratio of about 0.05 to 0.30, a low Volumetric Loading of about 5 to 30 pounds of BOD per day per 1000 cubic feet and a Recycle Ratio of about 0.75 to 1.50.
High rate aeration activated sludge process typically operates at higher MLSS levels of 4000 to 10000 mg/L but with a low dissolved oxygen level of about 1 to 3 mg/L, a more conventional solids retention time of 5 to 10 days. The high rate aeration process is also characterized by hydraulic retention time of only 2 to 4 hours, a high Food to Microorganism ratio of about 0.40 to 1.50, a high Volumetric Loading of about 100 to 1000 pounds of BOD per day per 1000 cubic feet and a high Recycle Ratio of between about 1.0 and 5.0.
The Covered Basin—High Purity Oxygen (HPO) activated sludge process is characterized as a system, such as a UNOX™ or OASES™ system, that employs a covered basin and direct injection of high purity oxygen in the mixed liquor within the covered basin to achieve higher dissolved oxygen rates of between about 2 to 20 mg/L. The covered basin systems typically operates at moderate MLSS levels of 2000 to 5000 mg/L and a very short solids retention time of 3 to 10 days. The Covered Basin-HPO process is also characterized by hydraulic retention time of only 1 to 3 hours, a Food to Microorganism ratio of about 0.25 to 1.00, a high Volumetric Loading of about 100 to 200 pounds of BOD/day per 1000 cubic feet and a Recycle Ratio of between about 0.25 to 0.50.
Many of the activated sludge processes, described above, produce large amounts of waste sludge that require further treatment and disposal. Treatment and disposal of excess sludge from wastewater treatment plants typically account for between about 25-65% of the total plant operation cost. The economic significance of this problem is increasing, due to more stringent regulations and rising disposal costs.
Existing methods for dealing with the removal of sludge include transporting the sludge to landfills, utilization of sludge for land application or agricultural purposes, and incineration of the sludge. In many regions, sludge disposal in landfills is being phased out and land application of sludge is becoming more strictly regulated to prevent environmental and health risks due to pathogens and toxic compounds in the sludge. Likewise, incineration of sludge is an expensive process and presents potential air pollution hazards. Due to the regulatory, environmental, and cost issues associated with solids handling and disposal, it is beneficial to minimize the amount of excess sludge produced in a wastewater treatment process.
Another major cost in the above-described wastewater treatment operations is electrical power. Current aeration systems used in wastewater treatment plants typically represent more than 50% of the overall plant power consumption. Power costs are increasing substantially due to rapidly rising electricity rates, and many electric power utilities have targeted wastewater treatment plants for possible electric power demand reductions.
Therefore, there is a significant need to reduce wastewater treatment plant operating costs through reduced solids handling and disposal costs together with power cost savings.